The present invention relates to a high-energy super-fine milling apparatus. More particularly this invention concerns such an apparatus intended to reduce particles to the nanometer range.
A standard such high-energy and/or superfine miller, also known as an attritor, has a milling vessel having a side wall centered on a vessel axis, defining a closed compartment, having a pair of end walls, and formed with a closable fill/empty port. An agitator rotatable about the vessel axis in the compartment has a shaft extending through one of the end walls and a plurality of arms projecting generally radially from the shaft. A drive motor adjacent the vessel and connected to the shaft rotates the agitator in the vessel about the vessel axis. The vessel is supported on a stationary frame. Such devices are used to reduce particulate material to extreme fineness for producing paint, alloying powder, and materials used in the semiconductor industry.
The standard prior-art system has the vessel mounted upright, with the vessel axis vertical. Filling such a system through the upper wall is particularly easy but it has the disadvantage of producing particles of nonuniform size. The kinetic energy of the agitator is effective perpendicular to gravity. Thus the distribution of the bodies being milled which have a relatively great mass is in part determined by gravity so that the kinetic density decreases from the bottom to the top of the load. This effect is particularly troublesome with wet milling, when liquid is added to the bodies being milled, since the liquid increases the mass being moved.
Accordingly commonly owned U.S. Pat. No. 5,464,163 describes such a milling apparatus having a milling vessel forming a milling chamber having a horizontal axis and adapted to receive a loose filling of milling bodies and a bearing-and-seal unit defining an end wall of the chamber, centered on the axis and provided with a shaft seal and a journal bearing. This bearing-and-seal unit has means enabling a number of different milling vessels to be juxtaposed with the bearing-and-seal unit. A shaft rotatable about the axis and sealed relative to the milling vessel by the shaft seal is journaled for rotation in the journal bearing. A rotor connected to the shaft and disposed in the milling vessel is formed with agitator elements imparting intensive movement to the bodies of the filling upon rotation of the shaft.
Another such horizontal axis machine is described in commonly owned U.S. Pat. 6,019,300 which has inlet and outer fittings open tangentially into a generally cylindrical milling vessel containing a charge of loose milling bodies. Gas flow through the vessel can alternate in velocity and the rotor speed can alternately pass from a relatively high speed to a relatively low speed.
Both these arrangements have certain advantages. Principally they produce a very uniform product, that is with a particle size lying in a very narrow range. Nonetheless it has been determined the high kinetic energy of the system is disadvantageous when working with sensitive materials. Furthermore emptying and filling the device is quite difficult, especially in a wet-milling system.
It is therefore an object of the present invention to provide an improved high-energy/superfine milling apparatus.
Another object is the provision of such an improved high-energy/superfine milling apparatus which overcomes the above-given disadvantages, that is which produces a highly uniform product even when working with sensitive materials, yet which is easy to fill and use, even for wet-milling.
A high-energy/superfine milling apparatus has according to the invention a milling vessel having a side wall centered on a vessel axis, defining a closed compartment, having a pair of end walls, and formed with a closable fill/empty port. An agitator rotatable about the vessel axis in the compartment has a shaft extending through one of the end walls and a plurality of arms projecting generally radially from the shaft. A drive motor adjacent the vessel and connected to the shaft rotates the agitator in the vessel about the vessel axis. In accordance with the invention the vessel, agitator, and drive motor are supported on a stationary frame for pivoting about a frame axis transverse to the vessel axis between an upright position with the vessel axis generally vertical and a horizontal position with the vessel axis generally horizontal.
With the milling apparatus according to the invention it is possible to dry- and wet-mill, respectively in the horizontal and vertical positions. The system allows simple milling, intensive mixing, dispersing, as well as mechanical alloying, high-energy milling, and reactive milling under wet and dry conditions with one and the same apparatus. It is possible to control the energy input to the vessel extremely accurately so that particularly sensitive materials that must have a particularly homogenous particle geometry and size can be produced with excellent results. The prior-art problems in mixer-type ball mills when producing pigments are largely avoided.
According to the invention the vessel is pivotal with the agitator and drive motor between the upright position and a pair of oppositely offset angled positions. More particularly the means for pivoting oscillates the vessel, agitator, and drive motor back and forth about the frame axis between the pair of offset angled positions that are offset by about 45xc2x0 to vertical. Highly sensitive metallic flakes, for instance of platinum, silver, or tantalum, can thus be produced since they will not drop to the bottom as in a standard vertical system. To allow this, the vessel is provided at its center of mass with a horizontal pivot shaft lying on and defining the frame axis.
In accordance with the invention a guide on the frame allows displacement displacing at least the other end wall and side wall of the vessel between an upper position and a lower position therebelow. In the lower position the other end wall and side wall are separated from the one end wall, drive motor, and agitator. In addition a gripper is provided for holding the one end wall, drive motor, and agitator on the frame above the other end wall and side wall in the lower position thereof. In addition the other end wall and side wall are tippable about a horizontal axis in the lower position. Thus the drive and agitator assembly, along with the one end wall, can be held while the side wall and other or bottom wall are lowered. Once lowered the vessel can be dumped out or exchanged with another vessel. A seal is provided between the one end wall and a rim of the side wall and releasable clamps allow the one end wall to be hermetically secured to the side-wall rim. Even if the system is used for horizontal-axis milling, the ability to turn it on end and set down the vessel makes loading and unloading it very easy.